Typical gaseous reduction systems incorporating vertical shaft, moving bed iron ore reduction reactors are disclosed in U.S. Pat. Nos. 3,765,872; 3,770,421; 3,779,741; and 3,816,102. In such systems reduction of the ore has commonly been effected by a reducing gas largely composed of carbon monoxide and hydrogen obtained by the catalytic reformation of a mixture of natural gas and steam. Such systems typically comprise a vertical shaft reactor having a reducing zone in the upper portion thereof and a cooling zone in the lower portion thereof. The ore to be reduced is fed to the top of the reactor and flows downwardly therethrough, first through the reducing zone wherein it is brought into contact with hot reducing gas and then through a cooling zone wherein it is cooled by a gaseous coolant before being removed at the bottom of the reactor. Effluent gas from the reducing zone is cooled to remove water therefrom, and in most cases a major part of the cooled effluent gas is reheated and recycled to the reducing zone. Similarly, at least a part of the cooling gas withdrawn from the cooling zone is commonly cooled and recycled to the cooling zone to cool the sponge iron to, or close to, the ambient temperature. At its lower end the reactor is provided with some means for controlling the discharge of cooled sponge iron from the reactor, e.g., a rotary discharge valve, a vibratory chute, conveyor belt or the like.
Sponge iron that is to be used in steel-making should desirably contain from 1% to 4% carbon, preferably in the form of ferric carbide rather than elemental carbon, such as soot. While a certain amount of carburization occurs in the reduction zone of reactors of the type described above, most of the required carburization has previously been effected in the cooling zone. For example, when using a non-stoichiometric reformer of the type shown in U.S. Pat. No. 4,370,162 as a source of make-up reducing gas, the carburization of sponge iron in the reduction zone is likely to be no more than about 0.5%. Additional carburization necessary to bring the sponge iron up to e.g. 2% carbon has been achieved by introducing methane, which may be in the form of natural gas, into the cooling gas that is circulated through the cooling zone of the reactor.
It is often desirable to discharge the sponge iron at high temperature for direct delivery to a steel-making furnace or, when the hot spong iron cannot be used immediately, to a melter or to a machine for briquetting the hot sponge iron produced in such a reactor. Direct reduced iron in the form of briquettes is easier to handle and transport than when it is in the form of the more porous, friable, and smaller sponge iron particles. Also briquetting of the sponge iron reduces the tendency for the iron to re-oxidize when stored in contact with atmospheric air. However, as noted above, the conventional reactors deliver the sponge iron at a temperature at or near ambient temperature. Densification of the product by briquetting at such low temperatures is difficult to achieve, requires high pressures, tends to produce friable products and an undesirably high proportion of fines. As a practical matter the sponge iron should be relatively near the sintering temperature to produce briquettes having acceptable physical properties.
More particularly, briquetting of the sponge iron should preferably be done at above about 700.degree. C. Below this temperature for most types of sponge iron the pressures typically required are too great and the wear of the briquetting equipment becomes excessive. Also, it becomes too difficult to densify the sponge iron adequately. A briquette with a density of at least 5 gm/cm.sup.3 is desirable for convenience of handling and for passivation. Dense briquettes have decreased exposed surface, lower porosity, and are less prone to fragmentation and fines generation.
The production of sponge iron at a temperature suitable for hot briquetting, typically 600.degree. to 800.degree. C., is not just a matter of operating the cooling zone of a conventional reactor at a higher temperature. As pointed out above, the cooling zone of a reactor performs both a cooling function and a carburizing function; a proper balance of the flow rate, composition and temperature of the cooling gas must be maintained to achieve this dual function. With the reactor operating conditions previously employed it has not been possible to produce sponge iron having both the optimum temperature for hot briquetting and the desired degree of carburization.